In the study of electronic materials and processes for fabricating such materials into an electronic structure, a specimen of the electronic structure is frequently used for microscopic examination for purposes of failure analysis and device validation. For instance, a specimen of an electronic structure such as a silicon wafer is frequently analyzed in a scanning electron microscope (SEM) and transmission electron microscope (TEM) to study a specific characteristic feature in the wafer. Such a characteristic feature may include the circuit fabricated and any defects formed during the fabrication process. An electron microscope is one of the most useful pieces of equipment for analyzing the microscopic structure of semiconductor devices.
In preparing specimens of an electronic structure for electron microscopic examination, various polishing and milling processes can be used to section the structure until a specific characteristic feature is exposed.
As device dimensions are continuously reduced to the sub-half-micron level, the techniques for preparing specimens for study in an electron microscope have become more important. The conventional methods for studying structures by an optical microscope cannot be used to study features in a modern electronic structure due to the unacceptable resolution of an optical microscope.
Although TEM techniques can provide a higher resolution image and a more detailed description of the internal structure of a specimen than is available using SEM techniques, they are only effective for electron transparent samples. Thus it is a basic requirement for TEM samples that the sample must be thin enough to be penetrated by the electron beam and thin enough to avoid multiple scattering, which causes image blurring. Nonetheless, it is recognized in the art that thin samples extracted from wafers may be brittle, and subject to fracture or crumbling. Furthermore, the fragile nature of thin extracted samples means that processes for extracting thin samples are difficult to automate, thus hindering efforts to automate these processes. In addition, TEM sample preparation and TEM imaging processes are usually time consuming and cannot be done in-line. In this process, the TEM sample has to be prepared, lifted-out from the wafer and put on a TEM sample holder, before it is ready for TEM imaging.
A dual column system, incorporating a scanning electron microscope and a focused ion beam (FIB) unit, can produce high resolution SEM images of a localized cross section. Typical FIB units are those manufactured by Applied Materials (Applied Materials, Santa Clara, Calif.) including the SEMVision™ G2 FIB and those available from FEI Company of Hillsboro, Oreg., including models 200, 820, 830, or 835. The skilled practitioner is referred also to U.S. Pat. No. 6,670,610 of Shemesh et al, titled “System and Method for Directing a Miller.” A typical dual column system includes a SEM column, a FIB column, a supporting element that supports the wafer and a vacuum chamber in which the wafer is placed while being milled (by the FIB column) and while being imaged (by the SEM column).
The cross section of a wafer is produced by: (i) locating a location of interest that should be milled in order to expose a cross section of the wafer, in which the locating is usually found by navigation of the SEM and sometimes also an optical microscope, (ii) moving the wafer (by a mechanical supporting element) so that the wafer is located under the FIB unit, and (iii) milling the wafer to expose the cross section. The cross-section is exposed by forming a small hole in the wafer (usually sized a few microns to few tens of microns in lateral and vertical dimensions). The cross section is usually vertical, so that the SEM should be tilted in order to image the cross section.
Today the resolution of a cross-section image generated by an SEM is limited to a few nanometers. The resolution is limited due to the charging effects of non-conductive (or partially conductive) materials of the wafer. The resolution is also limited due to the relatively large volume that emits electrons in response to an interaction with a charged particle beam. This volume is also referred to as an information volume.
It is noted that advanced FAB processes involve thin layers beyond the SEM resolution limit. In addition, the cross-section may include portions made of materials that cannot be distinguished by SEM imaging. For example, different types of dielectric layers appear on the SEM image with similar gray level, so that they are practically irresolvable.
The resolution of cross section images can be improved and the distinction between materials (also referred to as contrast) can be improved by various prior art processes.
One prior art process that improves the resolution and the material distinctiveness is complex and time consuming. It includes the following stages: (i) cross sectioning the wafer by breaking the wafer into samples, (ii) polishing the wafer sample of interest up to the required surface, (iii) performing wet etching by immersing the wafer sample in a solution (for example HF), (iv) coating the cross section with conductive material (such as Gold or Chrome of about 1 nanometer) and (v) imaging the (now coated) cross section.
This prior art process provides a cross section that has a fine topography that distinguishes between different materials (during the wet etch process different materials are etched at a different rate), and is coated with a conductive material so as to reduce charging effects and reduces the information volume (which is substantially limited to the conductive layer).
This mentioned above process has a few drawbacks, such as but not limited to the following: (i) wet etch cannot be executed within the vacuum chamber of a SEM (or of a dual column tool); (ii) the breaking of the wafer is destructive; and (iii) the overall process is complex and time consuming as the wafer has to be broken, polished, placed in a wet etch chamber, etched, removed from the wet etch chamber, placed into a material deposition chamber, coated with material, removed from the material deposition chamber, placed into the vacuum chamber of the SEM, and imaged.
Other techniques for milling a wafer are illustrated in US patent application publication serial number 2005/0103746 of Nadeau et al. and in US patent application publication serial number 2007/0093044 of Rijpers et al.
There is a growing need to provide fast and efficient methods and systems for imaging a cross section of a specimen.